fmt.Fprintln

Learn to write to a Writer with Fprintln.

fmt.Fprintln

The Fprint family writes output to any io.Writer: a file, an HTTP response, a buffer or os.Stderr.

fmt.Fprintln(os.Stderr, "uyarı:", err)
fmt.Fprintf(w, "<h1>%s</h1>", baslik)  // http.ResponseWriter

Each returns the number of bytes written and an error. When writing to a file or the network this error matters; the disk may fill up or the connection may drop.

Aligned output with tabwriter

text/tabwriter is a Writer that aligns tab-separated text into columns; ideal for command-line tables:

tw := tabwriter.NewWriter(os.Stdout, 0, 0, 2, ' ', 0)
fmt.Fprintln(tw, "AD\tFİYAT")
fmt.Fprintln(tw, "Kalem\t12.50")
fmt.Fprintln(tw, "Defter\t7.00")
tw.Flush()

Nothing is written unless Flush is called at the end.

Exercise Table output

Write an aligned table to os.Stdout with text/tabwriter: header AD\tADET, rows Kalem\t3 and Defter\t12. Use NewWriter(os.Stdout, 0, 0, 2, ' ', 0) and call Flush.

Show example solution

package main

import (
    "fmt"
    "os"
    "text/tabwriter"
)

func main() {
    tw := tabwriter.NewWriter(os.Stdout, 0, 0, 2, ' ', 0)
    fmt.Fprintln(tw, "AD\tADET")
    fmt.Fprintln(tw, "Kalem\t3")
    fmt.Fprintln(tw, "Defter\t12")
    tw.Flush()
}

Test yourself

What is the risk of writing user data directly with <code>Fprintf</code> when generating HTML?
- It's slower
- It doesn't compile
- There is no risk
- XSS: data isn't escaped, scripts can run
Show answer

XSS: data isn't escaped, scripts can run — fmt knows nothing about HTML; html/template or html.EscapeString must be used for user data.

Write to the error stream

fmt.____(os.Stderr, "hata:", err)

Show answer

fmt.Fprintln(os.Stderr, "hata:", err)

Writer target

os.Stderr

Error messages
http.ResponseWriter

Web response
tabwriter

Aligned table
bytes.Buffer

Collecting in memory

Next lesson: Summary - fmt package

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